If you use the Credentials Module and define a <credentials> section like this:
but you didn't add a credentials type for hcat in oozie-site.xml; that is, you did not add this:
then Oozie will silently not use the credentials class (because it doesn't know about it) so the action trying to use it will fail. It's pretty easy to forget to add the property to oozie-site.xml, so it would be nice if instead of silently ignoring needing to acquire a credential for an action, the workflow should fail either on that action when it tries to use the credential and can't find it, or (probably better) when submitting the workflow as part of the initial checking (e.g. after we do the forkjoin checking or something).